Two genetically distinct classes of meropenem-low-susceptibility Pseudomonas oaeruginosa PA02152 mutants, which arose spontaneously, were isolated. Two meropenem resistance genes, mpmA and mpmB, were mapped near ilvB/C and proC, respectively, on the P. aeruginosa PAO chromosome. The mpmA was thought to be identical to oprD2 because of the cross-resistance to carbapenems and the association with the loss of the outer membrane protein D2 (OprD2). The mpmB mutation conferred a 4-fold increase in resistance to meropenem, and cross-resistance to various types of antimicrobial agents, e.g. carbenicillin, norfloxacin and chloramphenicol. However, the mpmB mutant was susceptible to imipenem. This mutant still possessed OprD2 and showed increased expression of a 48-kD outer membrane protein, although its profiles of β-lactamase activity and affinities of penicillin-binding proteins for β-lactams were indistinguishable from those of the parent strain. The resistance gene mpmB was considered to be an allele of nalB (or cfxB or oprK) from the results of the transductional analysis. The mutation frequency of mpmA:mpmB was in the ratio of 4:1. The same results were obtained in another clinically isolated P. aeruginosa strain. Meropenem resistance caused by both mpmA and mpmB mutations seemed to be due to the reduction in permeability of antibiotics through the outer membrane. These findings suggest a new pathway for the translocation of meropenem other than that mediated by OprD2 across the outer membrane. Thus, meropenem showed about 4- to 8-fold higher activity than imipenem against OprD2-deficient P. aeruginosa.